This renewal application will deepen and extend our understanding of the effects of syringe exchange programs (SEP) by examining the phenomenon of secondary syringe exchange. Secondary syringe exchange (SSE) is the provision of syringes obtained from SEPs by injection drug users (IDUs) to other IDUs who do not attend SEPs. In addition, we will conduct and evaluate a peer mobilization intervention based on SSE networks. This research builds upon nine years of ongoing research with IDUs in the communities of Oakland and Richmond, CA (NIDA grants U01-DA06908 and R01-DA09532). Our studies have demonstrated the dramatic impact of SEPs on reducing the prevalence of high-risk injection practices. However, we have also found that many IDUs do not directly access SEPs, but rather obtain syringes from other IDUs who use SEPs. This practice is increasing among IDUs and extends the reach of SEPs, allowing sterile syringes to be distributed to many more IDUs than those who attend the programs. However, our data suggest that IDUs who receive syringes indirectly from SEPs through SSE were more likely to engage in high-risk injection practices than those who use SEPs directly and even those who do not use SEPs at all. The purpose of this grant is to better understand SSE practices and relationships, and to build upon these naturally occurring social networks with a peer mobilization intervention. The specific aims are as follows: (1) To investigate the prevalence, motivations and social contexts of secondary exchange relationships through a mixture of qualitative and quantitative research methods; (2) To prospectively assess levels of injection risk behaviors and HIV and HCV infection among IDUs engaging in secondary exchange relationships of different kinds; and (3) To develop, implement, and evaluate a social network-based peer mobilization intervention aimed at engaging secondary syringe exchange providers as peer educators and training them to provide education, motivation, and support for risk reduction among their secondary syringe exchange recipients.